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P0537 – A/C Evaporator Temperature Sensor Circuit Low (OBD2)

P0537 means "A/C Evaporator Temperature Sensor Circuit Low". The voltage the control unit measures in the air conditioning system circuit is below the permitted minimum. For a sensor this usually means the signal wire is shorted to ground or the reference voltage is missing; for an actuator, that its control wire is shorted to ground.

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P0537 quick summary

Fault codeP0537
MeaningA/C Evaporator Temperature Sensor Circuit Low
SystemPowertrain (Engine & Transmission)
VehiclesGeneric OBD2
SeverityLow
Can I drive?Yes. Only the air conditioning is affected.
Repair cost€20 – €600

How the engine control unit manages the air conditioning

The engine control unit monitors the air conditioning refrigerant pressure and evaporator temperature and switches the compressor clutch; these codes cover those sensors and circuits.

The A/C refrigerant pressure sensor tells the control unit the pressure in the high-pressure side. It is used to protect the compressor (too low or too high pressure), to control the cooling fan and to raise the idle speed when the compressor loads the engine. The evaporator temperature sensor prevents the evaporator from freezing.

Codes P0530–P0534 cover the pressure sensor and refrigerant charge loss; P0535–P0539 the evaporator temperature sensor; P0645–P0647 the compressor clutch relay.

If the control unit sees a fault, it switches the compressor off for protection – the A/C blows warm. Low refrigerant charge, faulty sensors and relay problems are common causes.

On VAG vehicles (VW, Audi, Škoda, Seat, Cupra), VCDS and older VAG testers can show this code as the five-digit VAG code 16921 (P0537). Both numbers mean the same fault.

Symptoms of P0537

  • Air conditioning blows warm
  • Compressor does not engage or cycles rapidly
  • Cooling fan runs constantly
  • Idle drops when A/C is switched on
  • Check Engine light on (some vehicles)

Most common causes of P0537 (most likely first)

Mechanic's Pro Tip

Never top up an A/C system without finding the leak – a recharge with UV dye or a leak test saves repeated refills and protects the compressor.

  1. 1.Signal or control wire shorted to ground

    Most likely

    Look for chafed insulation where the harness touches the engine, exhaust brackets or body edges.

  2. 2.Missing 5 V reference or supply voltage

    An open reference wire or a shorted sensor elsewhere on the same reference circuit can pull it down.

  3. 3.Internal short in the air conditioning system

  4. 4.Low refrigerant charge (leak)

  5. 5.Faulty refrigerant pressure sensor

  6. 6.Faulty evaporator temperature sensor

  7. 7.Failed compressor clutch relay or clutch coil

Step-by-step diagnostic guide

  1. 1

    Read codes and A/C pressure

    Read all codes and check the A/C pressure value in live data with the engine off and running.

  2. 2

    Measure the signal voltage

    With the ignition on and the air conditioning system plugged in, back-probe the signal wire. Then unplug it and check the reference/supply and ground wires separately.

    Multimeter: Signal near 0 V = short to ground or no reference. Reference wire: about 5 V with ignition on. Signal wire to ground with connector unplugged: no continuity.

  3. 3

    Check the refrigerant charge

    Have the A/C system checked with a service station – the amount of refrigerant recovered shows whether there is a leak.

  4. 4

    Check the pressure sensor

    Check the 5 V reference, ground and signal.

    Multimeter: With the engine off, the A/C pressure should correspond roughly to the outside temperature (several bar when warm).

  5. 5

    Check the compressor clutch

    Check the relay and the clutch coil resistance and air gap.

  6. 6

    Repair and recharge

    Repair leaks, replace faulty parts and recharge with the correct refrigerant amount.

Frequently asked questions

Can I keep driving with P0537?

Yes. Only the air conditioning is affected.

How much does it cost to fix P0537?

Typical repair costs are 20 € – 600 €, depending on the cause and the vehicle. The most common fixes are: signal or control wire shorted to ground; missing 5 V reference or supply voltage; internal short in the air conditioning system. Always confirm the cause with the tests above before buying parts.

Why does the engine control unit monitor the A/C?

Because the compressor loads the engine. The control unit raises the idle, controls the fan and protects the compressor.

Can I drive with an A/C code?

Yes. Only the cooling is affected.

What is the difference between P0537 and similar codes?

P0537 belongs to a group of codes for the same component that differ in the type or location of the fault: P0534 – Air Conditioner Refrigerant Charge Loss; P0535 – A/C Evaporator Temperature Sensor Circuit; P0536 – A/C Evaporator Temperature Sensor Circuit Range/Performance; P0538 – A/C Evaporator Temperature Sensor Circuit High; P0539 – A/C Evaporator Temperature Sensor Circuit Intermittent; P0645 – A/C Clutch Relay Control Circuit. The tests on this page apply to the whole group; the type of fault tells you where to start.

Related fault codes

This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.